Simultaneous control of exhaust emissions and thermal efficiency is an established goal in engine design. Optimization of engine design is limited by Cycle by Cycle Variability (CBCV), especially for spark ignition engines. CBCV is observed as either variations in the pressure diagram or as variations in flame propagation between consecutive engine cycles. In the vehicle the consequent unsteadiness in delivered engine power results in uneven vehicle progress which has been termed surge. Combustion variations require compromises in engine design, the setting of mixture composition and spark timing. This reduces engine power and efficiency at full load in order to meet roughness, noise, and octane requirements and at part load and idle reduces fuel economy and increases exhaust emissions in order to control surge.
If CBCV could be eliminated, the engine would run at its best economy settings and still produce a smooth and steady output. In addition, the fuel octane requirements could be reduced, or the compression ratio raised, with a consequent improvement in efficiency. Further, the lean limit of engine operation could be extended, resulting in a reduction in exhaust emissions and an improvement in thermal efficiency. It has been shown that the reduction of CBCV in lean-burn engines, together with control of ignition timing, can reduce NOx emissions and at the same time improve engine thermal efficiency. Another important benefit arising from control of cycle variations is the reduction in engine surge and improved vehicle driveability while cruising.
Much research has been conducted on lean-burn engines with the intention of improving efficiency and reducing emissions. The benefits from the lean combustion approach can be theoretically explained as follows. The excess air improves the engine's thermal efficiency by increasing the overall specific heats+ ratio, by decreasing the energy losses from dissociation of the combustion products, and by reducing the thermal losses to the engine cooling system. In addition, as the flame temperature drops with decreasing fuel air ratio, the NOx production is exponentialy reduced and the excess air may promote a more complete reaction of CO and hydrogen fuel emission from crevices and quench layers.
It is concluded that at the present state of development, U.S. emission standards present a considerable challenge to the realization of the fuel economy advantages theoretically inherent in lean burn engines. On the other hand, even though the incentives for lean burn application to automotive engines are valid and have good theoretical foundation, its implementation is a complex problem that requires several conflicting requirements to be satisfied simultaneously. Lean burn operation increases the CBCV and deteriorates vehicle driveability. CBCV increases with increasing air-fuel ratio.
Many attempts have been made to improve combustion efficiency. Such attempts include fuel stratification with a rich mixture in the spark plug region, divided or pre-chamber engines alone or in combination with stratification, and hydrogen enrichment of the whole fuel charge. None of these attempts have been entirely successful and the problems referred to above remain in evidence.
In the case of non-fuelled divided chamber engines, including the Bosch spark plug patented around 1978, the size (volume, connecting passage length and aperture) of the pre-chamber can only improve combustion at a particular power output. Thus, while combustion efficiency can be improved at a given power output, energy tends to be lost at full power to the pre-chamber walls and other parts of the main chamber by the impinging jet so that the peak power was reduced by about 10%. Furthermore, since the pre-chamber in the prior art arrangements is unfuelled, relying on the transfer of a fuel mixture from the main chamber, starting in cold conditions can be difficult.
In a paper entitled "High Chemical activity of incomplete combustion products and a method of pre-chamber torch ignition for avalanche activation of combustion in internal combustion engines" by L. A. Gussak of the Institute of Chemical Physics Academy of Sciences of the USSR, Moscow. (Publication No. 750890 of Society of Automotive Engineers USA) the author discusses the effects of pre-chamber torch ignition on the flame front of a hydrocarbon-air mixture and concludes that optimization is achieved by employing a pre-chamber volume of two to three percent of the compressed combustion chamber volume. While this paper contains some scientific consideration of the combustion products resulting from pre-chamber combustion of a very rich air-hydrogen mixture, the author does not come to any conclusion concerning the likelihood of pre-chamber combustion providing a significant benefit in the improvement of engine thermal efficiency while at the same time reducing NOx emissions.
The Patent literature also contains some reference to the burning of hydrogen in pre-chambers, the most pertinent prior art being U.S. Pat. No. 4,140,090 Lindburg and U.S. Pat. No. 4,760,820 Tozzi. The Lindburg reference provides a small pre-chamber for burning hydrogen but specifically teaches the introduction of an oxidant to be mixed with the hydrogen fuel to ensure stoiciometric proportions. The reference is also silent at the nature of the exit passage. The present applicant has found that the mixture in the pre-chamber should preferably be hydrogen rich and the outlet orifice should be carefully dimensioned to ensure that a proper ignition jet stream issues from the orifice to ensure complete combustion of a lean fuel mixture in the combustion chamber.
In the case of the Tozzi reference, the magnetic field generating means introduces undesirable complexity and increased power consumption to generate plasma temperatures of around 4,000.degree. to 6,000.degree. C. Plasma igniters of the type described by Tozzi have not employed commercial success presumably due to the complexity and power consumption difficulties involved.